As an electric motor, a brushless slotless motor is used, for the sake of the suppression of rotation unevenness and vibration (for example, Patent Literature 1).
A plurality of coils are disposed at the inner circumference of a cylindrical stator core of the motor, and a rotor in which a permanent magnet having the number of poles corresponding to the number of the coils is provided at the periphery of a shaft is disposed at the inner circumference of the coils.